core/blockchain: Change iterator in procFutureBlocks

[holiman]

to use lru.Peek instead of Get

See Get versus
Peek :

Returns the key value (or undefined if not found) without updating the "recently used"-ness of the key.

Without this change, we're reordering the recentness stochastically when processing future blocks.

[mention-bot]

@holiman, thanks for your PR! By analyzing the history of the files in this pull request, we identified @fjl, @obscuren and @karalabe to be potential reviewers.